Des McMorrow

Low dimensional quantum spin systems High-temperature superconductivity Multipolar order in novel materials Contact details: Office: Lab 1 Room 3C4 Tel: +44 (0)20 7679 7189 Ext: 37189 Fax: +44 (0)20 7679 0595 Email:d.mcmorrowucl.ac.uk

My research is focussed on understanding how electrons organise themselves in solids to produce the wonderfully diverse range of phenomena encountered in modern condensed matter physics. I am particularly interested in how low-dimensionality and strong collective quantum effects can produce new states of matter, such as the high-temperature superconductors, quantum spin liquids, etc. The techniques I use are based mainly on using x-rays and neutrons to probe the structural and magnetic correlations that dominate the low-energy behaviour of these and other interesting classes of solids. This invariably means that I spend significant periods of time at central facilities such as ISIS in Oxfordshire, and the Institute Laue-Langevin and the European Synchrotron Radiation Facility, both located in Grenoble, France .

Presently I teach one of the MSci/MSc options, “Order and excitations in Condensed Matter Physics”. The blurb I wrote for the course reads: “Condensed matter physics not only underpins much of modern technologies, but provides model systems in which to develop and test new theoretical concepts that find wider application in other areas of physics This course aims to provide an overview of how we can understand the wonderfully diverse range of ordering phenomena displayed by modern materials. Both structural and magnetic aspects of the ordering will be examined and explained, with examples taking us from magnetic nano-particles and devices used in computer storage devices to mayonnaise! One major theme of the course is to explain the theory and practice of modern experimental techniques used in the study of order and excitations in solids. This includes a one-day trip to Rutherford Appleton Laboratory near Oxford where currently £700m is being spent building the next generation of neutron and x-ray sources.”
One of my other main responsibilities is the recruitment of PhD students for the CMMP group.

• Quantum phase transition of a magnet in a spin bath. H.M. Ronnow et al. Science. 308, 389 (2005).

The excitation spectrum of a model magnetic system, LiHoF₄, was studied with the use of neutron spectroscopy as the system was tuned to its quantum critical point by an applied magnetic field. The electronic mode softening expected for a quantum phase transition was forestalled by hyperfine coupling to the nuclear spins. We found that interactions with the nuclear spin bath controlled the length scale over which the excitations could be entangled. This generic result places a limit on our ability to observe intrinsic electronic quantum criticality. 

• Dispersive Excitations in the High-Temperature Superconductor La_2–xSr _xCuO₄. N. B. Christensen et al. Phys. Rev. Lett. 93, 147002 (2004).

This paper relates to one of the key open questions in condensed matter physics, namely the mechanism producing the anomalously high superconducting transition temperatures in some cuprates. In particular it addresses the nature of the magnetic fluctuations and establishes that the magnetic fluctuations in different cuprate families are surprisingly universal in nature. Prior to this work it was widely held that the magnetism was fundamentally different in the various families, and hence it was not possible to construct a single theory based on a magnetically mediated pairing mechanism. This work was featured in an article in Physics Today, volume 57, Number 9, page 24, September 2004. [PDF file] 

• Elements of Modern X-ray Physics. Jens Als-Nielsen and Des McMorrow. John Wiley and Sons, 2001.

Biography
2004-Present           Professor of Physics, UCL
2004-2009               Royal Society Wolfson Merit Award
2002-2003               Senior Research Scientist, Riso National Laboratory
2000-2002               Head of Research Programme, X-ray and Neutron scattering, Riso
1998-2003               External lecturer in Physics, Niels Bohr Institute
1993-2000               Senior Scientist, Risø
1991-1993               Senior College Lecturer, Keble College, Oxford
1990-1993               Research Fellow, Clarendon Laboratory, University of Oxford
1983-1987               Ph. D., University of Manchester
1979-1983               B.Sc. in Physics, University of Sheffield

Research  
One of key challenges in trying to unravel the mysteries of the high-temperature superconductors is to find the correct starting description of the electron correlations in the normal state from which superconductivity emerges. Our approach to tackle this issue is to study the dynamic magnetic correlations of the electron spins using neutron scattering techniques. This  allows us to distinguish between different models and by comparing results from different families of superconductors it provides a method of understanding which features are peculiar to a given family, and which features are universal. The image summarises what was believed to be true about the spin dynamics in two of most studied families of superconductors prior to our recent experiments. It was widely accepted that the LSCO family could be understood in terms of a stripe model, in which the doped holes form one dimensional conducting rivers in a background of antiferromagnetically coupled spins. YBCO on the other hand was thought to be better described as a homogeneous Fermi liquid. The excitation spectrum was thought to reflect these differences as shown in (a) and (b). Our experiments in fact demonstrated that at low energies the excitations share the same gross structure, pointing to a much higher level of universality than had hitherto been appreciated.